Field of the Invention
The present invention relates to a halide glass used as an infrared-transmitting material. The halide glass of the present invention is used particularly in a glass fiber for long distance optical communication, an infrared optical fiber for thermometer, a window for infrared laser, etc.
Description of the Prior Art
Fluorozirconate glasses composed mainly of zirconium fluoride (Japanese Patent Publication No. 24349/1986) and fluoroaluminate glasses composed mainly of aluminum fluoride are known as an infrared-transmitting material having a good transmitting property over an ultraviolet to near-infrared or middle-infrared region. Mixed halide glasses (Japanese Patent Application Kokai (Laid-Open) No. 264344/1985) are also known as said material.
Of these glasses, fluorozirconate glasses have relatively excellent devitrification resistance as fluoride glasses and can easily be made into a fiber; however, they have poor chemical durability and insufficient mechanical strength. Meanwhile, there have been reported fluoroaluminate glasses having excellent chemical durability and excellent mechanical strength; however, they have poor devitrification resistance and accordingly are difficult to make them into an optical fiber.
The present inventors found a novel fluoride glass composed mainly of aluminum fluoride, having (1) excellent chemical durability and excellent mechanical strength and moreover, unlike the conventional fluoride glass composed mainly of aluminum fluoride, (2) such thermal stability as to enable the fiber drawing, and filed a patent application therefor (Japanese Patent Application No. 36382/1987). This fluoride glass, however, is very liable to cause unhomogeneous nucleation and consequently crystallization from the glass surface; this invites in some cases crystallization from the surface when the glass is made into a fiber, and the resulting fiber tends to have scattering loss by the crystal. The glass forming range of this glass is narrow at which the glass has such stability as to enable the fiber drawing When an optical fiber composed of a core glass and a clad glass is produced from said glass by changing the glass compositions for the core glass and the clad glass to allow them to have different refractive indices, the stability of the glass has been sacrificed